Love these videos!! But I'm confused. So many questions... I think I'm asking the same question in many different ways. I hope somebody can take the time to enlighten me. I see two things that we can actually observe and measure... 1) We are using 7591A tubes 2) We measured the turn ratio of 24:1 on the power transformer. So we start there. I follow the math easily enough, but what is it actually telling us? Why are we assuming the secondary impedance is 8 ohms? Is that because we will connect that to an 8 ohm speaker? Maybe the entire circuit from the output tubes all the way through to the speaker is looking for an 8 ohm impedance at the transformer?? Do we KNOW that the secondary impedance is 8 ohms, or are we saying that this is what the circuit SHOULD have? I suppose we could measure the DC resistance of the secondary winding (out of circuit), but that's very different from the impedance, right? Can we measure the actual impedance? If we do assume the secondary impedance is 8 ohms, then according to the formula the primary impedance is 4,608 ohms... so if the tubes want to see 6600 ohms, are they drawing too much current? Assuming the rest of the circuit is done correctly AND we use a transformer with the correct turn ration, then will the magnetic forces in the transformer just work themselves out to give us 8 ohms on the secondary and 6600 ohms on the primary? Using the formula and the back of an envelope (literally) I figure if we want 6600 ohms on the primary and 8 ohms on the secondary, we need a turn ratio of 28.72:1. So is that the answer... select a power transformer with that turn ratio and the tubes should be happy?
Correct on the speaker. It will be 8 ohms when hooked up (hopefully). Unless it isn't the original speaker. Then another calculation would be needed plugging in the desired speaker impedance. I'll give others a chance to answer some of the other questions. Pinned.
No one else wants to tackle some questions? DC resistance in a transformer coil is not the same as impedance. The impedance of a coil changes depending on the current, frequency, and voltage. Impedance is not static. When you hit a note on the guitar and current changes, the impedance also changes. This is a good video summarizing the math. Watch to the end: ruclips.net/video/WmTlioVfS78/видео.html
Not sure if this will create interest or not, but this guy shows a great way to measure impedance. The problem is as you stated, you can not measure impedance, it being an AC measure, like resistance, being a DC measure. Here is the link if you are interested.' www.learnabout-electronics.org/ac_theory/impedance73.php
Impedance is a complex subject. It's taking something from one dimension and moving into 2 (or even 3) dimensions. That's why Brad used a 60Hz AC "signal" from his variac instead of the DC measurement from his multimeter. 8 ohms is an assumption. But the most common speaker value outside of automotive applications. But the transformer suggest an output of 11 ohms - which would be a very uncommon speaker. As for the mis-match - you lose power. (Much like a car in the wrong gear. A transmission can be thought of as a "impedance matching" device that matches the RPM/torque of the engine to the RPM of the rear wheels.) I think it might work ok driving a 16 ohm speaker - trying to drive too high an impedance is just a loss of max power while driving too low of a one tends to make things struggle. If that was the only problem, it might be OK, but this amp has a load of them.
...at this point we are thinking to deep, and can't blame any component for the amp not working.. All we know is what the customer has said, which can be often misleading. No offense but the owner of the amp doesn't seem organized, look how the letter was written and the lazy packing job. ...to add to the questions...let's think of load lines...what happens if we rotate counterclockwise from the "book" load? Less current...but what else...bigger voltage swing and less linear operation...this can cause huge fly back voltage and also exceed the max plate voltage of the tube. What if we rotate clockwise from the "book" load? As the line becomes more vertical we risk screen failure. We do not have the be right at the load impedance that RCA tells us and that is not always the best sounding load line. Finally...the tubes in that amp will not match RCA's bogey tube...and if your screen voltage is different due to the unknown PT, everything is changed... Friendly rant, hopefully contributing to this discussion. Brad, I agree you made the right choice to walk away from this amp. If it was yours, you would have spent the time and turned it into a tone machine, stock or not.
You'll see a Boston pencil sharpener mounted to my bookcase behind me sometimes. When I got that thing, I immediately sharpened every pencil in the house. I was so SICK of not having sharp pencils. Plus there is something satisfying about sharpening a pencil. I think it goes back to elementary school. One teacher I had used to use that as a reward. The best kid got to sharpen all the #2 test pencils.
So was the Ken Fisher reference in the title intentional or not? (For those who may not get it: Ken Fischer was the man behind probably what most consider the Holy Grail of hand wired guitar amps- Trainwreck Amps. Early in Fischer’s career he was an engineer at Ampeg, leaving in ‘67. His biggest contribution at Ampeg was designing the unique tremolo circuit in the Gemini amps.)
Ah I get it, Amp WRECK. I have a Gemini VI from 1965 (in working condition, sounds glorious). Good to know the tremolo circuit was designed by Ken Fischer. Thanks.
Can't feel bad about this one Brad. You did all you could with out racking up a crazy bill and even showed the math that led to the conclusion. And you got a great lesson out out of it diagnosing transformers. That's a positive to end on brother.
Something related but unrelated . A bit of wd40 inside the nozzle and barrel of a solder sucker stops the solder sticking and clogging it up. Just apply when needed :) Great stuff man !! Love your channel !!
Since this amp uses a 6FQ7 to drive the reverb tank directly, and a 7199 P.I. That may also explain why things were rewired, although the 6FQ7's (6FG7) are inexpensive and easy to find. You are right however. To rework this amp. (it is so sloppy) You are basically going to rebuild the entire thing. Too Bad the original Output tranny is gone. The original OT is gorgeous on these, going down to 30 HZ with a clean waveform. The one I worked on needed a little redesign, .. it clipped on the negative going side of the signal at about 5 Volts. By adjusting the screen setup on the 7199, I was able to get a nice symmetrical waveform that clipped at about 22V, for both the plus and minus side of the signal.... Anyone working on these old amps, also, be aware that the eyelet board is just mounted with two screws to a narrow strip of wood. On this one.the large old filter Cap clips were removed leaving nothing to hold it onto the chassis. It is very easy to develop solder "icicles" that go right down to the chassis because most of the eyelets have only a little space between them and the metal chassis. Zapp Pop! The Joe Piazza circuit diagrams are the best!
Brad, my first thought when I saw the number of leads coming out of the primary on the output transformer was that someone has tried to retrofit an ultra-linear output transformer into this circuit. I could not see where the leads were going so...just a thought. As you already know ultra-linear output transformer are used in Hi-Fi tubes amp to help the frequency response of circuit maintain a more flat response...and not something we normally care about in guitar amps...if it were possible to look up the manufacturer and model of the transformer we could learn what it actually is.
Thanks for another video, Brad! I agree with your decision to skip on this one :) There is a bit easier and faster way of calculating the ratios and impedances for OTs. Once you've determined that the ratio is 24:1 and you presumed the primary impedance is 6600, you can square the 24, so 24x24=576. Then divide primary impedance 6600 with 576 and you get 11.46 ohms on the secondary. I guess it's easier to remember and simpler to calculate.
These comments are incredible, so much brainpower in the audience. I love old amps but I stay with the common tubes, the first thing I do when scoping out a cool amp is price out the tubes.
Wow what a wreck, and hack job was done on this amp, before You got it.. I just finished one in relatively good shape to begin with, but it did need quite a lot of work. Those two transformers are definitely both replacements. The main replacement filter cap is done completely wrong, as this and many other Ampegs have the ground floating, until it is powered on. With this setup the AC power would short to ground when the amp is SWITCHED OFF !,.. as the goofy setup with these is that the high side of power gets shorted to ground to discharge power caps when the amp is off!.. I always change the power switch to the plus side and change all appropropriate circuits. The main filter cap has to float, the can can't touch ground, unless the whole power switch configuration is adjusted !
The black and green seem to be a separate winding used only for feedback, black to ground, green to feedback resistor. Technically there's nothing wrong with that as long as the feedback ratio is correct which can be changed by adjusting the value of the 150K feedback resistor. But since you don't have the original OT you have no way knowing what the original winding ratio was or even if this OT has the correct impedance ratio for the tubes and speaker. But you can easily measure and calculate that by applying an AC signal to one of the windings and measuring the AC voltages on the other windings.
Oh, I see you already did that, suits me for replying before watching the rest of the video. An easier way to do that would be to take N1/N2=24/1=24, square that, multiply it with whatever secondary impedance you have, then take that primary impedance and look at the curves for the tube at your Vscreen and draw a load line. If it's not where you want it you can change the secondary impedance or screen voltage (if you can) to get you there. But even if it's off by 50%, tubes are very forgiving and it'll likely work fine if not overdriven hard into output stage clipping. Just make sure you stay under the class B 2*Pa anode dissipation limit. In your case Zpri=4k6 at Zsec=8. You'll be fine as long as you stay at or below the anode voltage spec'd for 6k6 in the datasheet.
Looking at the 7591 curves, the class-B Va=Vg2=350V (Za=1150ohm, Ia(0V)=300mA) load line sits well below the 2*Pa limit. At Va=Vg2=400V Ia(0V)=350mA it just touches the 2*Pa limit. So at 350V you're safe and at 400V you need to be absolutely sure you don't go under 8 ohms speaker impedance or it'll exceed the tube ratings. Bias it at 70% Pa for class-AB at 350V, and less at 400V (I'd stay under 50% Pa)
Brad you know a lot about tube amps, more than me. It is interesting how many people with similar knowledge view your channel! The comments that offer help are fantastic. You don't need help. You could do all that it takes to service this amp back to being a player amp. I learn from you and the comments. What ever happens to this amp, let us know, please! This is a tube amp soap opera! Thanxz
Bummer. I've had one of these on a back shelf for over 20 yrs and have been planning to pull it out and get to work on it very soon so I was stoked to stumble across this video but very sad it didn't go anywhere. I haven't compared them yet but am hoping there's enough similarities between this and the Gemini I that the video on that one will be helpful.
Ahhh...a day in the life of a tube amp tech. I got something in for repair very similar to this. It was a Bell Pacemaker PM-33, and completely trashed. He ended up just giving it to me in lieu of my $50 bench fee...fun times!
Maybe you should have, but, he did make it sound as though it was working?! From the looks of that thing, it has been non-functional for quite some time. Besides: do the folks sending in amps for repair even know, specifically, what to tell you? Very rarely in my experience. Anyway, keep up the good work...on that note, doesn't the turns ratio squared times the desired output impedance equal the primary impedance needed? In your case, with an 8 ohm load, the primary would have been seeing 4.6K. Might be right for a pair of 6L6s in a Vibrolux...
Hi Brad, about a month ago you were testing the NuTube(s) you received and you mentioned building a guitar amp with them. Have you gotten to play around with this idea? And if so have you had some good results?
You should consider teaching classes on this stuff. I suck at math, like golf ball through a garden hose level of suck, but this totally made sense and my eyeballs didn't glaze over immediately. Thumbs up.
Math is one of those things you have to practice a lot to stay good at. I don't practice enough to teach this stuff. I have to refer to the books myself.
Don't underestimate yourself. I build all my own amps and I love watching your videos. You could teach this. You just have to stay one step ahead of the students. You have the gift and the passion.
I had a 12inch. Crate that my tech told me it was toast. Too costly to repair. Two years later and I saw the same amp for sell in his shop. I considered buying it back.
I had an Ampeg Gemini in 1985 that was in good working order. Everybody liked to rip on me about the accordion input! I wish I still had it. It was a little bit different than the one you have. The output transformer was way different than this one. It was much smaller. The amp was only rated around 25 watts with those 7591's and was very clean sounding because of the feedback. Almost sounded like a hifi amp instead of a guitar amp. It had a single 15" Jensen speaker with a really stiff cone. Instead of the dimension control, mine had a reverb knob. tremolo sounded really good. Shame this one's so far gone. Thanks for the video!
Btw, about your calculations. It's a bit more convenient to calculate back to the primary side. The reason is that most output transforms are made for certain tubes. With calculating the primary side you can see much quicker what it was made for.
Wow, that is one clean and highly organized work bench ya have there. It must be nice to have a new/bigger space to work your magic (my inner A-type/ OCD personality is active today lol). I had to laugh when I saw the bolts used to install the new tube sockets. In my limited experience, such workmanship is usually a pre-cursor to a "dogs breakfast". Thanks for the video and knowledge!
poor thing... looks like a good learning experience for hobbyist because it would would take a lot of time tracking down specs for transformer, etc... it can be fixed! but it may take a a few full days of work and research and the amp may not be worth that even. as a tech myself i helped clients make this kind of decision at least twice today.
It hurts don't it.Nice old amp still looking half decent and then find out someone Frankensteind it,thinking they could make it work. Good money after bad on this one.Great if you had an extra week to redo it all.Another good one Brad.
Hey Brad, I understand the reason behind you not wanting to fix that turd but wanted you to know that it's a lot more entertaining watching you struggle with a complicated repair and then finally solving the problem. It allows you to show off your skills.
And also,,,, Like you, I'm a huge fan of Uncle Doug but he needs some help with his amp demos. I would love to see you help him demo his cool amps.... Now that would be awesome!!!
Your pessimism was very disturbing. Your measurements were very encouraging. I’m a tech at Ginters Guitar Shop in Madison Wisconsin. Send the amp to me I will fix it using the transformers on the chassis. I doubt I’ll have more than two hours in it. The amp is worth fixing and I like these kind of projects.
2 hours? Dude, by the time you realize I was right to pass on this, you'll already be 3 or 4 hours in. You're certainly welcome to it. :D What are you going to do first time the amp is treated roughly and the Philco radio PT rips out of the 3 mounts? How are you going to overcome the impedance mismatch on the OT? If you go to 6L6, how will you know if the speaker in the cabinet can handle the peak power? If you just run the 7591, you're going to be operating the amp at reduced efficiency. You've got 2x replaced sockets, God knows why. It's blowing fuses, God knows why. Who knows if the opto-coupler is fried. I didn't test that. Man, this is one of those that could just end up being one thing after another. I've seen it before and chased the rabbit down the hole and regretted it.
I work for a reputable, real shop. My name is Fred, just like my youtube name. guitarshopofwisconsin.com/ I am willing to take this on. It's not uncommon that I work on something longer than I feel I can bill for. Short term that sounds like a poor business plan but long term it works out just fine. 50 year old amps often come with a challenge but it is worth it to save this bit of amplifier history. That's what we do at the Guitar Shop of Wisconsin. We have probably the largest selection of old Ampegs in the Midwest. Honestly, don't send that back to your customer, Have him contact us.....He can mail it here. Thanks
It's POSSIBLE that the extra windings on the transformer are to go to the Output tube screens. "Ultra Linear" type output transformer. That sounds like a very ampeg thing to do. It's a Hi-Fi style transformer. I've got one I plan to use on a bass amp. Oh sure... then show the Schematic to us........ Ultra Linears would have 5 leads on the input side.
You can plug 4 or 16 ohms into that equation and see if it gets you closer, yes. And one of them will. But that would mean changing the speaker then. Kinda a pick your poison situation between replacing the OT, changing output tube type, or changing speakers, or even some combination of those.
Greetings Brad, i love your videos and thank you for all of them. IMHO i believe you could have checked the other leads of the transformer, maybe the winding would be different and you could have using those windings, not saying I know better, Just a prayer for a miracle XD, also, maybe the customer could have sourced a 2nd hand transformer, again, prayer for miracle, anyway, really sorry you could not service the amp, look like a beautiful amp. best wishes for you and your family
This is my life (as a 15 year old now).... Me: I have to study for school, I will get an F again. Also Me: Oh heck Brad has uploaded a new video finally I have something to do.
You inspired me in electronics (as well as Uncle Doug) and got me interested in it so much that I started exploring more and got invited to participate in the regional competition in electronics in Croatia which is a big thing for kids like us here...
I think you should of tried, tell him you need up to a year cause your busy and give him the cost of transformer. Would of made a good sets of rebuild videos.
Brad, Thank you for turning a "not worth it to fix it" video into an educational video showing how to measure the estimated transformer turns ratio & impedance in/out. I learned from that and hope to do some simiilar tests on my 1st tube amp troubleshooting & repair. It's a shame that we won't get to watch you troubleshoot & repair this amp, but I think you're correct in stopping work on it due to the probable cost in $$$ & your time as a Tech. The owner will probably agree with you stopping work on it, unless it has some kind of sentimental value for him/her.
Thanks. Yeah, I messaged the customer and he agreed. He's going to do a rebuild on it as a learning experience, I think. He's a budding tinkerer and this will be a good one to learn on.
At 14:00 that is the bias supply diode. and the bias cap 100 mfd at 100V . Turns out there is about 19 volts on it, and it is derived from B+ winding.. The output Tranny now, is an ultra linear,. and seems also to be wired up wrong because the Green and brown wires are the taps that should go to the screen grids on the Output tubes or tied off, if not used. Someone may have tried to convert this amp to run EL34's or 6L6,s
We are sending you these puzzles and challenges because we love you...we want you to make money...lol. Anyways, although you decided this amp is a no-go it was very educational. It thought me there can be situations in which a repair is not worth the energy and money ...unless the customer insists of course.
When you connected the multimeter to the output transformer's secondary to count the windings, did you connect one lead to the center tap? It's supposed to be 8ohm from one output to center, not end to end...right?
Excellent vid, in a lot of ways, for me at least, more educational tgan everything going "right" - love to watch more like this, even though it's a pain in the arse for you. Excellent.
Hey Brad, I agree with your transformer math, no hives, but the 8 ohms is at the center tap. Over all would it be closer to 16 ohms. Moot point I guess since you sent it packing but I was wondering. Thanks and keep up the great videos.
if you put 6L6GC or EL34 output tubes they have a 5000ohm impedance, and that would work out to 8.6ohms so that transformer would work for those. power tube pinouts are the same. minimal effort to ensure it is biased right.
100 in parts to fix that amp, worth it. Anyone thinks this one is bad, they should see my echo twin. I will never get it fixed, it is way beyond me. Not sure why I bought it?
Hey brad if you don’t mind me asking how do you advertise your work and do you hold any licenses? Meaning like I’m mepc certified “low voltage electrician” and where did you learn that math ?
Unrelated...being unbias and subjective. David Koresh wasn't a bad guitar player. I only say this because I was listening to his music right before I clicked on this video. I'm always fascinated by these dynamics. Much like Brad doing superb work but struggling with his sexual identity. 😂😂😂
This channel is really all about recruiting. The compound is almost complete and the final standoff is at hand. Unfortunately, Janet Reno is no longer around, cause I'd like to send her a very strongly worded letter.
Did you know they used bunker buster to kill everyone in the concrete section of the compound? The place where the woman and children where hiding from the shootout
I would have called n said" I can get this running so you can use it for $###. Or if you want it 100% multiply that by four.See if he will go with A or B. Me personally? Get it running.I'd knock it off in an evening on the bench.
Bench fee, just document the errors and advise owner sell it to a hobbyist to sort out. The chassis, especially if it has a cabinet and speaker are worth it as a dIY project base.
Bummer man! I was so hoping that would go well. I have a 67 just like that, that is near mint. It needs attention and was hoping to pick up a few pointers. I can assure you it that inside it looks nothing like that abomination. I'd feel comfortable to say mine is 99% original with only the 2 20uf caps and the 220K 1W resistor in the power section replaced. The resistor was a known bad part that I read Ken Fisher say was routinely replaced at the factory back in the day since it was way under rated in that position and almost always fried. To be certain those are not the original transformers. The replacement can cap looks like it might be a PCB mount. Fliptops.net sells a cap kit that includes a clamp like that, I believe the original 80/40/40 is riveted in. Mine has the CTS square magnet speaker. With that speaker (they were also available with a JBL D130) and the bullet proof cabinet, id be willing to wager it weighs more than a Fender Twin Reverb. I had to really dig, but was able to locate a copy of the original Ampeg factory schematic. Unlike the Piazza version, which is laid out much differently and maybe more logically, the factory issue includes the voltages. I can share it with you if you're interested. If so, message me.
It has been touched by multiple people. My customer only did the caps and rectifier diodes, I think. Which were not that bad. They needed some touch up, for sure, but I'd encourage him to keep plugging away and keep improving.
Man that wiring looks like my first DIY project. Obviously we all gotta start somewhere but damn, at least clip the hanging leads so you don't end up with high voltage shorting to the chassis. I kinda hope the customer does agree to pay because I bet you'd find some interesting gremlins in there.
One thing I'll say is at least he took a valiant stab at it! That's a lot more than some people might have done. I don't expect perfection when it's a first project.
for real, when the options are between "first job pretty bad, but learned a ton, so second job is way better" and "sit, do nothing, and learn nothing" then the choice is obvious.
Output transformer would be fine for 7591s or 6L6GCs. Measures 4.6k ohms, close enough. original was 6.6k. Would be perfect for 6l6s, a common mod when 7591s were hard to get. Send it to me, I’ll fix it. 😀
Yes, it would work with 6L6. I'm not saying it wouldn't operate with 7591. It would. But this transformer is not ideal. Either speaker impedance would need to change or amp rebiased so the tubes operate where they should. Moving to 6L6 makes little sense also because I'm not sure what speaker he's using. If it's the original speaker, it's possible it couldn't handle the peak power of a pair of 6L6GC. If you want to work on it, more power to ya. :D
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Love these videos!! But I'm confused.
So many questions... I think I'm asking the same question in many different ways.
I hope somebody can take the time to enlighten me.
I see two things that we can actually observe and measure...
1) We are using 7591A tubes
2) We measured the turn ratio of 24:1 on the power transformer.
So we start there.
I follow the math easily enough, but what is it actually telling us?
Why are we assuming the secondary impedance is 8 ohms? Is that because we will connect that to an 8 ohm speaker? Maybe the entire circuit from the output tubes all the way through to the speaker is looking for an 8 ohm impedance at the transformer??
Do we KNOW that the secondary impedance is 8 ohms, or are we saying that this is what the circuit SHOULD have?
I suppose we could measure the DC resistance of the secondary winding (out of circuit), but that's very different from the impedance, right? Can we measure the actual impedance?
If we do assume the secondary impedance is 8 ohms, then according to the formula the primary impedance is 4,608 ohms... so if the tubes want to see 6600 ohms, are they drawing too much current?
Assuming the rest of the circuit is done correctly AND we use a transformer with the correct turn ration, then will the magnetic forces in the transformer just work themselves out to give us 8 ohms on the secondary and 6600 ohms on the primary?
Using the formula and the back of an envelope (literally) I figure if we want 6600 ohms on the primary and 8 ohms on the secondary, we need a turn ratio of 28.72:1. So is that the answer... select a power transformer with that turn ratio and the tubes should be happy?
Correct on the speaker. It will be 8 ohms when hooked up (hopefully). Unless it isn't the original speaker. Then another calculation would be needed plugging in the desired speaker impedance. I'll give others a chance to answer some of the other questions. Pinned.
No one else wants to tackle some questions?
DC resistance in a transformer coil is not the same as impedance. The impedance of a coil changes depending on the current, frequency, and voltage. Impedance is not static. When you hit a note on the guitar and current changes, the impedance also changes. This is a good video summarizing the math. Watch to the end: ruclips.net/video/WmTlioVfS78/видео.html
Not sure if this will create interest or not, but this guy shows a great way to measure impedance. The problem is as you stated, you can not measure impedance, it being an AC measure, like resistance, being a DC measure.
Here is the link if you are interested.'
www.learnabout-electronics.org/ac_theory/impedance73.php
Impedance is a complex subject. It's taking something from one dimension and moving into 2 (or even 3) dimensions. That's why Brad used a 60Hz AC "signal" from his variac instead of the DC measurement from his multimeter.
8 ohms is an assumption. But the most common speaker value outside of automotive applications. But the transformer suggest an output of 11 ohms - which would be a very uncommon speaker. As for the mis-match - you lose power. (Much like a car in the wrong gear. A transmission can be thought of as a "impedance matching" device that matches the RPM/torque of the engine to the RPM of the rear wheels.) I think it might work ok driving a 16 ohm speaker - trying to drive too high an impedance is just a loss of max power while driving too low of a one tends to make things struggle. If that was the only problem, it might be OK, but this amp has a load of them.
...at this point we are thinking to deep, and can't blame any component for the amp not working.. All we know is what the customer has said, which can be often misleading. No offense but the owner of the amp doesn't seem organized, look how the letter was written and the lazy packing job.
...to add to the questions...let's think of load lines...what happens if we rotate counterclockwise from the "book" load? Less current...but what else...bigger voltage swing and less linear operation...this can cause huge fly back voltage and also exceed the max plate voltage of the tube.
What if we rotate clockwise from the "book" load?
As the line becomes more vertical we risk screen failure.
We do not have the be right at the load impedance that RCA tells us and that is not always the best sounding load line.
Finally...the tubes in that amp will not match RCA's bogey tube...and if your screen voltage is different due to the unknown PT, everything is changed...
Friendly rant, hopefully contributing to this discussion.
Brad, I agree you made the right choice to walk away from this amp. If it was yours, you would have spent the time and turned it into a tone machine, stock or not.
I love that you're using a Green Eggs and Ham pencil for impedance calculations.
You'll see a Boston pencil sharpener mounted to my bookcase behind me sometimes. When I got that thing, I immediately sharpened every pencil in the house. I was so SICK of not having sharp pencils. Plus there is something satisfying about sharpening a pencil. I think it goes back to elementary school. One teacher I had used to use that as a reward. The best kid got to sharpen all the #2 test pencils.
So was the Ken Fisher reference in the title intentional or not?
(For those who may not get it: Ken Fischer was the man behind probably what most consider the Holy Grail of hand wired guitar amps- Trainwreck Amps. Early in Fischer’s career he was an engineer at Ampeg, leaving in ‘67. His biggest contribution at Ampeg was designing the unique tremolo circuit in the Gemini amps.)
Ah I get it, Amp WRECK. I have a Gemini VI from 1965 (in working condition, sounds glorious). Good to know the tremolo circuit was designed by Ken Fischer. Thanks.
I love those Ampeg Gemini series amps. My main thang is a 66 Gemini II. 15" W 7591 output tubes. Great amp.
That’s a great example of doing the right thing for you and your customer
Can't feel bad about this one Brad. You did all you could with out racking up a crazy bill and even showed the math that led to the conclusion. And you got a great lesson out out of it diagnosing transformers. That's a positive to end on brother.
Thank you, Carl. Always good to hear from you sir.
When the power transformer burned up on my Sunn 200, it was a very expensive repair-now I see why-love your videos-thanks!
Something related but unrelated . A bit of wd40 inside the nozzle and barrel of a solder sucker stops the solder sticking and clogging it up. Just apply when needed :)
Great stuff man !! Love your channel !!
Since this amp uses a 6FQ7 to drive the reverb tank directly, and a 7199 P.I. That may also explain why things were rewired, although the 6FQ7's (6FG7) are inexpensive and easy to find. You are right however. To rework this amp. (it is so sloppy) You are basically going to rebuild the entire thing. Too Bad the original Output tranny is gone. The original OT is gorgeous on these, going down to 30 HZ with a clean waveform. The one I worked on needed a little redesign, .. it clipped on the negative going side of the signal at about 5 Volts. By adjusting the screen setup on the 7199, I was able to get a nice symmetrical waveform that clipped at about 22V, for both the plus and minus side of the signal.... Anyone working on these old amps, also, be aware that the eyelet board is just mounted with two screws to a narrow strip of wood. On this one.the large old filter Cap clips were removed leaving nothing to hold it onto the chassis. It is very easy to develop solder "icicles" that go right down to the chassis because most of the eyelets have only a little space between them and the metal chassis. Zapp Pop! The Joe Piazza circuit diagrams are the best!
Brad, my first thought when I saw the number of leads coming out of the primary on the output transformer was that someone has tried to retrofit an ultra-linear output transformer into this circuit. I could not see where the leads were going so...just a thought. As you already know ultra-linear output transformer are used in Hi-Fi tubes amp to help the frequency response of circuit maintain a more flat response...and not something we normally care about in guitar amps...if it were possible to look up the manufacturer and model of the transformer we could learn what it actually is.
Man I would love to see more Ampeg videos
Thanks for another video, Brad! I agree with your decision to skip on this one :) There is a bit easier and faster way of calculating the ratios and impedances for OTs. Once you've determined that the ratio is 24:1 and you presumed the primary impedance is 6600, you can square the 24, so 24x24=576. Then divide primary impedance 6600 with 576 and you get 11.46 ohms on the secondary. I guess it's easier to remember and simpler to calculate.
I never can remember the equations when it comes time to use them anyway. Anything outside Ohm's Law, and I need to bust out the books.
These comments are incredible, so much brainpower in the audience.
I love old amps but I stay with the common tubes, the first thing I do when scoping out a cool amp is price out the tubes.
Wow what a wreck, and hack job was done on this amp, before You got it.. I just finished one in relatively good shape to begin with, but it did need quite a lot of work. Those two transformers are definitely both replacements. The main replacement filter cap is done completely wrong, as this and many other Ampegs have the ground floating, until it is powered on. With this setup the AC power would short to ground when the amp is SWITCHED OFF !,.. as the goofy setup with these is that the high side of power gets shorted to ground to discharge power caps when the amp is off!.. I always change the power switch to the plus side and change all appropropriate circuits. The main filter cap has to float, the can can't touch ground, unless the whole power switch configuration is adjusted !
like the video. It shows how some amps can go very wrong and once they are mussed up it can't be fixed, so still a learning experiance.
Anything CAN be fixed, it's just a question of whether it's economical to fix.
The black and green seem to be a separate winding used only for feedback, black to ground, green to feedback resistor. Technically there's nothing wrong with that as long as the feedback ratio is correct which can be changed by adjusting the value of the 150K feedback resistor. But since you don't have the original OT you have no way knowing what the original winding ratio was or even if this OT has the correct impedance ratio for the tubes and speaker. But you can easily measure and calculate that by applying an AC signal to one of the windings and measuring the AC voltages on the other windings.
Oh, I see you already did that, suits me for replying before watching the rest of the video. An easier way to do that would be to take N1/N2=24/1=24, square that, multiply it with whatever secondary impedance you have, then take that primary impedance and look at the curves for the tube at your Vscreen and draw a load line. If it's not where you want it you can change the secondary impedance or screen voltage (if you can) to get you there. But even if it's off by 50%, tubes are very forgiving and it'll likely work fine if not overdriven hard into output stage clipping. Just make sure you stay under the class B 2*Pa anode dissipation limit.
In your case Zpri=4k6 at Zsec=8. You'll be fine as long as you stay at or below the anode voltage spec'd for 6k6 in the datasheet.
Looking at the 7591 curves, the class-B Va=Vg2=350V (Za=1150ohm, Ia(0V)=300mA) load line sits well below the 2*Pa limit.
At Va=Vg2=400V Ia(0V)=350mA it just touches the 2*Pa limit.
So at 350V you're safe and at 400V you need to be absolutely sure you don't go under 8 ohms speaker impedance or it'll exceed the tube ratings.
Bias it at 70% Pa for class-AB at 350V, and less at 400V (I'd stay under 50% Pa)
Brad you know a lot about tube amps, more than me. It is interesting how many people with similar knowledge view your channel! The comments that offer help are fantastic. You don't need help. You could do all that it takes to service this amp back to being a player amp. I learn from you and the comments. What ever happens to this amp, let us know, please! This is a tube amp soap opera! Thanxz
A lot of viewers of this channel know WAY more than I know. I always appreciate when they pipe up.
You certainly did a deep dive on this one! I would be interested in your customer's response. Maybe he will tell you to keep it !!!???
Bummer. I've had one of these on a back shelf for over 20 yrs and have been planning to pull it out and get to work on it very soon so I was stoked to stumble across this video but very sad it didn't go anywhere. I haven't compared them yet but am hoping there's enough similarities between this and the Gemini I that the video on that one will be helpful.
Ahhh...a day in the life of a tube amp tech. I got something in for repair very similar to this. It was a Bell Pacemaker PM-33, and completely trashed. He ended up just giving it to me in lieu of my $50 bench fee...fun times!
I refunded half the bench fee on this one because I felt like I should have asked a few more questions about the state of it before I had him ship it.
Maybe you should have, but, he did make it sound as though it was working?! From the looks of that thing, it has been non-functional for quite some time. Besides: do the folks sending in amps for repair even know, specifically, what to tell you? Very rarely in my experience.
Anyway, keep up the good work...on that note, doesn't the turns ratio squared times the desired output impedance equal the primary impedance needed? In your case, with an 8 ohm load, the primary would have been seeing 4.6K. Might be right for a pair of 6L6s in a Vibrolux...
Hi Brad, about a month ago you were testing the NuTube(s) you received and you mentioned building a guitar amp with them. Have you gotten to play around with this idea? And if so have you had some good results?
Need to clear out some customer amps and do a couple reviews I've promised to do, then I'll wire up some NuTubes and plug a guitar in.
Alex Chilton reportedly used a Gemini for the clean guitars on Big Star's Radio City.
The chorus to "The Gambler" started playing in my head about 8 minutes in...
I use a clip of that song in my last video I couldn't service.
The Guitologist Yeah, I remember!
Great video. It might be a good idea for those shipping equipment to you to take a photo before packing and or including notes in the letter.
You should consider teaching classes on this stuff. I suck at math, like golf ball through a garden hose level of suck, but this totally made sense and my eyeballs didn't glaze over immediately. Thumbs up.
Math is one of those things you have to practice a lot to stay good at. I don't practice enough to teach this stuff. I have to refer to the books myself.
Don't underestimate yourself. I build all my own amps and I love watching your videos. You could teach this. You just have to stay one step ahead of the students. You have the gift and the passion.
I’ll pledge $5 towards seeing that ampeg come back to life.
Call in the family as the doctor would say.
I had a 12inch. Crate that my tech told me it was toast. Too costly to repair. Two years later and I saw the same amp for sell in his shop. I considered buying it back.
Thanks Brad, once again a very informative video - I have written that equation down!
I had an Ampeg Gemini in 1985 that was in good working order. Everybody liked to rip on me about the accordion input! I wish I still had it. It was a little bit different than the one you have. The output transformer was way different than this one. It was much smaller. The amp was only rated around 25 watts with those 7591's and was very clean sounding because of the feedback. Almost sounded like a hifi amp instead of a guitar amp. It had a single 15" Jensen speaker with a really stiff cone. Instead of the dimension control, mine had a reverb knob. tremolo sounded really good. Shame this one's so far gone. Thanks for the video!
xraytonyb sounds like a Gemini II (G-15). My current favorite amp. The tremolo was responsible for the "Crimson and Clover" effect...
Love the videos you put up Tony
Ampegs were designed to be "hi-fi" by the way
thats why I was always allied with Marshall
xraytonyb Ironic that they ripped on you about the accordion input, that's the best sounding input for a guitar on these!
Funny but true.
And this is why I am learning more about this stuff. I love the puzzle!
You're in the right place if you like puzzles.
Btw, about your calculations. It's a bit more convenient to calculate back to the primary side. The reason is that most output transforms are made for certain tubes. With calculating the primary side you can see much quicker what it was made for.
In this case it would be around 4600k for the tubes. Sound like a transformer for some EL34 or 6550.
Wow, that is one clean and highly organized work bench ya have there. It must be nice to have a new/bigger space to work your magic (my inner A-type/ OCD personality is active today lol). I had to laugh when I saw the bolts used to install the new tube sockets. In my limited experience, such workmanship is usually a pre-cursor to a "dogs breakfast". Thanks for the video and knowledge!
poor thing... looks like a good learning experience for hobbyist because it would would take a lot of time tracking down specs for transformer, etc... it can be fixed! but it may take a a few full days of work and research and the amp may not be worth that even. as a tech myself i helped clients make this kind of decision at least twice today.
I was enjoying this until math happened...I broke out into hives. Lol. JK, another great video.
Me too at times. I love his videos though and it amazes me how these guys can figure out what has gone wrong..lol.
5 leads should be for a Hi-Fi ultra linear output transformer. I bet it wasn't the original transformer obviously. I hope you can make it sound great!
It hurts don't it.Nice old amp still looking half decent and then find out someone Frankensteind it,thinking they could make it work. Good money after bad on this one.Great if you had an extra week to redo it all.Another good one Brad.
This is one of those where if he had sent the whole amp I would just offer to buy it and restore later on my own time.
Hey Brad, I understand the reason behind you not wanting to fix that turd but wanted you to know that it's a lot more entertaining watching you struggle with a complicated repair and then finally solving the problem. It allows you to show off your skills.
Thanks for saying so. I still feel bad for the owner and the amp itself. I know what it COULD be given enough money, time and energy.
And also,,,, Like you, I'm a huge fan of Uncle Doug but he needs some help with his amp demos. I would love to see you help him demo his cool amps.... Now that would be awesome!!!
Your pessimism was very disturbing. Your measurements were very encouraging. I’m a tech at Ginters Guitar Shop in Madison Wisconsin. Send the amp to me I will fix it using the transformers on the chassis. I doubt I’ll have more than two hours in it. The amp is worth fixing and I like these kind of projects.
2 hours? Dude, by the time you realize I was right to pass on this, you'll already be 3 or 4 hours in. You're certainly welcome to it. :D
What are you going to do first time the amp is treated roughly and the Philco radio PT rips out of the 3 mounts? How are you going to overcome the impedance mismatch on the OT? If you go to 6L6, how will you know if the speaker in the cabinet can handle the peak power? If you just run the 7591, you're going to be operating the amp at reduced efficiency. You've got 2x replaced sockets, God knows why. It's blowing fuses, God knows why. Who knows if the opto-coupler is fried. I didn't test that. Man, this is one of those that could just end up being one thing after another. I've seen it before and chased the rabbit down the hole and regretted it.
I work for a reputable, real shop. My name is Fred, just like my youtube name. guitarshopofwisconsin.com/ I am willing to take this on. It's not uncommon that I work on something longer than I feel I can bill for. Short term that sounds like a poor business plan but long term it works out just fine. 50 year old amps often come with a challenge but it is worth it to save this bit of amplifier history. That's what we do at the Guitar Shop of Wisconsin. We have probably the largest selection of old Ampegs in the Midwest. Honestly, don't send that back to your customer, Have him contact us.....He can mail it here. Thanks
Thanx for uploading this oddball...
Entertaining and instructive....
have a sweet day
It's POSSIBLE that the extra windings on the transformer are to go to the Output tube screens. "Ultra Linear" type output transformer. That sounds like a very ampeg thing to do. It's a Hi-Fi style transformer. I've got one I plan to use on a bass amp. Oh sure... then show the Schematic to us........ Ultra Linears would have 5 leads on the input side.
heheheh...I got a lot of people in the comments with that one. :P
Also ( sorry i forgot to add) could the speaker be swapped for a 4ohms or 16 ohms to make the transformer compatible ?
So many prayers
You can plug 4 or 16 ohms into that equation and see if it gets you closer, yes. And one of them will. But that would mean changing the speaker then. Kinda a pick your poison situation between replacing the OT, changing output tube type, or changing speakers, or even some combination of those.
you would need a 12 ohm speaker for the OT to have a reflected impedance of 6.6k ...
Greetings Brad, i love your videos and thank you for all of them.
IMHO i believe you could have checked the other leads of the transformer, maybe the winding would be different and you could have using those windings, not saying I know better, Just a prayer for a miracle XD, also, maybe the customer could have sourced a 2nd hand transformer, again, prayer for miracle, anyway, really sorry you could not service the amp, look like a beautiful amp.
best wishes for you and your family
Yay! 😊i love these videos so much im gonna stand in the street naked sreaming "LONG LIVE THE GUITOLOGIST!"
Film or it didn't happen.
This is my life (as a 15 year old now).... Me: I have to study for school, I will get an F again. Also Me: Oh heck Brad has uploaded a new video finally I have something to do.
The Guitologist - inspiring failure and success at the same time.
You inspired me in electronics (as well as Uncle Doug) and got me interested in it so much that I started exploring more and got invited to participate in the regional competition in electronics in Croatia which is a big thing for kids like us here...
My life as a 36yo - I'm supposed to be studying for exams in May... Brad's ruining both of our lives as usual!!
You should be on the History Channel, keep up the good work.
I'm almost old enough.
In the words of the great Dave Rheaume himself... GOOCHED.
That one's beyond gooched. I think it may be fuckulated.
I think you should of tried, tell him you need up to a year cause your busy and give him the cost of transformer. Would of made a good sets of rebuild videos.
Brad, Thank you for turning a "not worth it to fix it" video into an educational video showing how to measure the estimated transformer turns ratio & impedance in/out. I learned from that and hope to do some simiilar tests on my 1st tube amp troubleshooting & repair. It's a shame that we won't get to watch you troubleshoot & repair this amp, but I think you're correct in stopping work on it due to the probable cost in $$$ & your time as a Tech. The owner will probably agree with you stopping work on it, unless it has some kind of sentimental value for him/her.
Thanks. Yeah, I messaged the customer and he agreed. He's going to do a rebuild on it as a learning experience, I think. He's a budding tinkerer and this will be a good one to learn on.
A very good and educational video.
it almost feels like someone sent you a "test "amp ,.... juuust to see if you could find the wrong transformer
and i think you passed ;)
Yeah, like Pat Sajack or Richard Dawson will jump out now and give me my reward.
I just thought someone was fucking with him . Maybe just to see if he could fix it .
All over my head but looks a real head scratchier bro. I guess some things in this world were never meant to be.
It happens. Only the second time I've had to pass on one in channel history.
At 14:00 that is the bias supply diode. and the bias cap 100 mfd at 100V . Turns out there is about 19 volts on it, and it is derived from B+ winding.. The output Tranny now, is an ultra linear,. and seems also to be wired up wrong because the Green and brown wires are the taps that should go to the screen grids on the Output tubes or tied off, if not used. Someone may have tried to convert this amp to run EL34's or 6L6,s
This was some good entertainment! Much respect to your knowledge and skills!
Thanks, man. Always hate to do it, but sometimes the best advice is to cut losses and flip it to a hobbyist who will make a labor of love out of it.
We are sending you these puzzles and challenges because we love you...we want you to make money...lol. Anyways, although you decided this amp is a no-go it was very educational. It thought me there can be situations in which a repair is not worth the energy and money ...unless the customer insists of course.
Old Ampegs never die.
When you connected the multimeter to the output transformer's secondary to count the windings, did you connect one lead to the center tap? It's supposed to be 8ohm from one output to center, not end to end...right?
Correct. I used the center to end to measure the winding used for the speaker itself, not the whole secondary end to end.
Thanks for clarifying.
Excellent vid, in a lot of ways, for me at least, more educational tgan everything going "right" - love to watch more like this, even though it's a pain in the arse for you. Excellent.
Hey Brad, I agree with your transformer math, no hives, but the 8 ohms is at the center tap. Over all would it be closer to 16 ohms. Moot point I guess since you sent it packing but I was wondering. Thanks and keep up the great videos.
We are only concerned with the actual reflected load that will be present when the amp is operating, hence the reason we used the 8 ohm tap.
10 minutes into this video This amp looks like a beginning hobbyist build
Good call on that one. And the video was worth it for the transformer check. :-)
I hope u get that off him and make a video of you redesigning it
Looks like the OT colored wires are the secondaries with different ohm taps?
Confusing really. I thought that was possible too, but then I wondered why they would be going back into the amp board...
Killer amps!!, Hey everybody you can bridge both channels together too .
Ampeg part # OT-215 Model 8950015 Made by Electrical Windings for Ampeg. Ampeg SB12, Gemini II, GV and other amps.
So how did the customer react to the news? What he/she understanding?
Yeah, he understood. No probs.
Looks like the last order I got from ebay... Kitchen trash makes great packing material right?
After watching the video, Im guessing he used the contents of the dumpster he found that amp in to ship it...
if you put 6L6GC or EL34 output tubes they have a 5000ohm impedance, and that would work out to 8.6ohms so that transformer would work for those. power tube pinouts are the same. minimal effort to ensure it is biased right.
I love your videos. I was impressed also by your transistor/cap tester. Where did you buy that?
There is a link in the video description.
100 in parts to fix that amp, worth it. Anyone thinks this one is bad, they should see my echo twin. I will never get it fixed, it is way beyond me. Not sure why I bought it?
Hey brad if you don’t mind me asking how do you advertise your work and do you hold any licenses? Meaning like I’m mepc certified “low voltage electrician” and where did you learn that math ?
Unrelated...being unbias and subjective. David Koresh wasn't a bad guitar player. I only say this because I was listening to his music right before I clicked on this video. I'm always fascinated by these dynamics. Much like Brad doing superb work but struggling with his sexual identity. 😂😂😂
This channel is really all about recruiting. The compound is almost complete and the final standoff is at hand. Unfortunately, Janet Reno is no longer around, cause I'd like to send her a very strongly worded letter.
The Guitologist hahaha! We must finish writing the seals Brad.
Did you know they used bunker buster to kill everyone in the concrete section of the compound? The place where the woman and children where hiding from the shootout
Bill Knott David Koresh ???!.
Why are you guys talking about my dad
Look at the big brain on Brad. Busted out the algebra. See kids, you may actually usebit
Good find, transformers are getting harder and harder to find
i'm wondering if it was used for parts by someone else before the owner got it.
excellent video! I enjoyed every minute!
That's a shame. I had a Gemini II. It was a fun amp.
Thank you for posting !
A good amp tech knows when to say no.
Break out the butter n jelly cuz that amp is toast.
Looks like an old H+K cream machine type or a Peavey that came out in the 80s-90s.
it's just the square root of Z1 / Z2 no? easy peasy - that gives your number of turns factor between N1 and N2
Good video mate big thumbs up 👍
I would have called n said" I can get this running so you can use it for $###. Or if you want it 100% multiply that by four.See if he will go with A or B. Me personally? Get it running.I'd knock it off in an evening on the bench.
24x24x8 = 4608 --> so this is a 4.4k:8 ohm OT (for Fender 6L6's)
Brings to mind a quote from an old Monty Python movie: "Run away! Run away!!" :-O
Bring up the Holy Hand Grenade of Antioch! ruclips.net/video/tgj3nZWtOfA/видео.html
The only thing worse than getting me started on Monty Python quotes would be moving on to Firesign Theater! LOL
Install a parts kit for Princeton. Better than new!
What a bummer. Usually you can make things like this work. James Gang "Walk Away" seems fitting, unfortunately.
Gaiam yoga blocks FTW!!!
it could work well with a 16 ohm speaker ?
Bench fee, just document the errors and advise owner sell it to a hobbyist to sort out. The chassis, especially if it has a cabinet and speaker are worth it as a dIY project base.
Basket case - throw money at it - or walk away and call it a day.
Bummer man! I was so hoping that would go well. I have a 67 just like that, that is near mint. It needs attention and was hoping to pick up a few pointers. I can assure you it that inside it looks nothing like that abomination. I'd feel comfortable to say mine is 99% original with only the 2 20uf caps and the 220K 1W resistor in the power section replaced. The resistor was a known bad part that I read Ken Fisher say was routinely replaced at the factory back in the day since it was way under rated in that position and almost always fried. To be certain those are not the original transformers. The replacement can cap looks like it might be a PCB mount. Fliptops.net sells a cap kit that includes a clamp like that, I believe the original 80/40/40 is riveted in. Mine has the CTS square magnet speaker. With that speaker (they were also available with a JBL D130) and the bullet proof cabinet, id be willing to wager it weighs more than a Fender Twin Reverb. I had to really dig, but was able to locate a copy of the original Ampeg factory schematic. Unlike the Piazza version, which is laid out much differently and maybe more logically, the factory issue includes the voltages. I can share it with you if you're interested. If so, message me.
You can email me that schematic for next time. Thanks. bradlinzy at gmail
Does no one clean a terminal after soldering?! A little alcohol on an acid brush (or q-tip if you're desperate) goes a long way.
Somebody worked on that who should never ever touch any form of electronics ever again!!!!
It has been touched by multiple people. My customer only did the caps and rectifier diodes, I think. Which were not that bad. They needed some touch up, for sure, but I'd encourage him to keep plugging away and keep improving.
Yeah it was the rest of it that was pretty sketchy
Everyone has to start somewhere. Fortunately, my beginning was long before RUclips.
Nice!! (the video, not the result)
Man that wiring looks like my first DIY project. Obviously we all gotta start somewhere but damn, at least clip the hanging leads so you don't end up with high voltage shorting to the chassis. I kinda hope the customer does agree to pay because I bet you'd find some interesting gremlins in there.
One thing I'll say is at least he took a valiant stab at it! That's a lot more than some people might have done. I don't expect perfection when it's a first project.
for real, when the options are between "first job pretty bad, but learned a ton, so second job is way better" and "sit, do nothing, and learn nothing" then the choice is obvious.
Daniel G .
Oh man, it's a shame. Ampeg Geminis are really good sounding amps in my opinion. But just looking at this is painful
Ampegs are killer, yes.
Output transformer would be fine for 7591s or 6L6GCs. Measures 4.6k ohms, close enough. original was 6.6k. Would be perfect for 6l6s, a common mod when 7591s were hard to get.
Send it to me, I’ll fix it. 😀
Yes, it would work with 6L6. I'm not saying it wouldn't operate with 7591. It would. But this transformer is not ideal. Either speaker impedance would need to change or amp rebiased so the tubes operate where they should. Moving to 6L6 makes little sense also because I'm not sure what speaker he's using. If it's the original speaker, it's possible it couldn't handle the peak power of a pair of 6L6GC. If you want to work on it, more power to ya. :D
Nonsense, they’re not making any more of these. Someone is going to fix it.
Sometimes the towel throws you.
Most everything in this world can be fixed by someone. The real question is are they worth the time and money vs buying a new one.